Propagation Time Measurement Device and Electronic Key System

1. A propagation time measurement device that measures a propagation time of a transmission signal from when the transmission signal is transmitted, through wireless communication, from a base station to a terminal to when the transmission signal is returned to the base station from the terminal, wherein a plurality of continuous waves having different frequencies are combined to generate the transmission signal, the propagation time measurement device comprising: a correlation computation unit that generates a plurality of correlation values by performing a correlation computation on each of the continuous waves and a reception signal that is returned from the terminal and received by the base station; and a propagation time measurement unit that measures the propagation time from a phase of the correlation value generated for each continuous wave in the correlation computation.

2. The propagation time measurement device according to claim 1 , wherein each of the continuous waves is a complex signal including a real part and an imaginary part.

3. The propagation time measurement device according to claim 2 , further comprising: a real number acquisition portion that generates the transmission signal by acquiring a plurality of real parts from the continuous waves and combining the real parts of the continuous waves; a first demodulation unit that generates a real part of the reception signal by demodulating the reception signal at a predetermined phase; a second demodulation unit that generates an imaginary part of the reception signal by demodulating the reception signal at a phase retarded by 90° from the predetermined phase; and a complex signal generation unit that generates a complex signal based on the real part, generated by the first demodulation unit, and the imaginary part, generated by the second demodulation unit, wherein the correlation computation unit is one of a plurality of correlation computation units, and each of the correlation computation units generates the correlation value by performing a correlation computation on a complex signal of a corresponding continuous wave and the complex signal generated by the complex signal generation unit.

5. The propagation time measurement device according to claim 1 , wherein the transmission signal transmitted from the base station includes a fixed frequency band.

6. The propagation time measurement device according to claim 1 , wherein the propagation time measurement unit estimates the propagation time based on an amount of change in a phase of the reception signal from a phase of the transmission signal.

7. The propagation time measurement device according to claim 1 , further comprising: an expected value computation unit that generates a plurality of complex correlation values by performing an expected value computation on the correlation values from the correlation computation unit; a vector unit that vectorizes the complex correlation values from the expected value computation unit; a correlation matrix unit that acquires a correlation matrix of the vectorized complex correlation values from the vector unit; and an average frequency computation unit that acquires an average frequency of the correlation matrix from the correlation matrix unit, wherein the propagation time measurement unit estimates the propagation time based on the average frequency from the average frequency computation unit.

8. An electronic key system in which a transmission signal is transmitted, through wireless communication, from a vehicle to an electronic key and returned to the vehicle from the electronic key, wherein a plurality of continuous waves having different frequencies are combined to generate the transmission signal, the electronic key system comprising a propagation time measurement device that measures a propagation time of the transmission signal from when the vehicle transmits the transmission signal to the electronic key to when the transmission signal returns to the vehicle from the electronic key, wherein the propagation time measurement device includes a correlation computation unit that generates a plurality of correlation values by performing a correlation computation on each of the continuous waves and a reception signal that is returned from the electronic key and received by the vehicle, and a propagation time measurement unit that measures the propagation time from a phase of the correlation value generated for each continuous wave in the correlation computation.